Method of producing high yield pulp by disc refining at ph of 12 to 14

ABSTRACT

BONDING BETWEEN THE FIBERS IN FIBER BUNDLES OF HIGH YIELD PULP PRODUCED BY HOT DEFIBRATION BECOMES WEAKENED AND DISRUPTED WITHOUT CAUSING ANY LONGITUDINAL SPLITTING OR TRANSVERSAL BREAKING OF FIBERS, WHEN THE MATERIAL IS IMPREGNATED WITH AN AQUEOUS ALKALINE MEDIUM HAVING A PH OF FROM 12 TO 14 AND THE IMPREGNATED MATERIAL IS SUBJECTED TO COMPRESSIVE STRESS SUBSTANTIALLY PERPENDICULAR TO THE COMMON DIRECTION OF THE FIBER AXES IN THE BUNDLES AND SIMULTANEOUSLY TO SHEARING STRESS SUBSTANTIALLY IN THAT DIRECTION, SAID SHEARING STRESS BEING SUFFICIENT AT LEAST TO LOOSEN THE BOND BETWEEN THE FIBERS OF THE BUNDLE. THE TREATED MATERIAL MAY THEREAFTER BE SUBJECTED TO A CONVENTIONAL BEATER OR REFINER TREATMENT SO AS TO FIBRILLATE THE INDIVIDUAL FIBERS COMPLETELY, IF NECESSARY, AND STRONG PAPER CONTAINING NO FIBER BUNDLES CAN BE MADE FROM THE SO TREATED MATERIAL.

US. Cl. l6225 lint. Cl. Dill) 1/16 3 Claims ABSTRACT 9F THE DECLEUSURE Bonding between the fibers in fiber bundles of high yield pulp produced by hot defibration becomes weakened and disrupted without causing any longitudinal splitting or transversal breaking of fibers, when the material is impregnated with an aqueous alkaline medium having a pH of from 12 to 14 and the impregnated material is subjected to compressive stress substantially perpendicular to the common direction of the fiber axes in the bundles and simultaneously to shearing stress substantially in that direction, said shearing stress being sufficient at least to loosen the bond between the fibers of the bundle. The treated material may thereafter be subjected to a conventional heater or refiner treatment so as to fibrillate the individual fibers completely, if necessary, and strong paper containing no fiber bundles can be made from the so treated material.

This application is a continuation-inpart of the copending application Ser. No. 633,067, filed on Apr. 24, 1967 and application Ser. No. 795,737, filed on Jan. 31, 1969, now abandoned.

This invention relates to a method of producing high yield pulp containing no fiber bundles. The term high yield pulp in the specification and claims means to be pulp having a yield of more than 60%.

Recently, utilization of high yield pulp has become increasingly important in the paper industry. Such high yield pulp, however, comprises numerous bundles of bonded fibers, and it is very difficult to separate the interbonded fibers in the fiber bundles into individual fibers by the conventional mechanical treatment, without causing any longitudinal splitting or transversal breaking of fibers.

If high yield pulp is subjected to the conventional heater or refiner treatment to such an extent that the fiber bundles disappear, the strength of paper made from the treated pulp will be very poor.

In respect of its suitability for printing and of appearance, the paper produced from paper stock containing fiber bundles is far inferior to the paper containing no fiber bundles.

In addition, the paper produced from high yield pulp is usually loW in strength, and high yield pulp had to be mixed with a great amount of sulfite or sulfate pulp. Therefore, a method of separating the fiber bundles in high yield pulp into individual fibers and simultaneously increasing the strength of paper produced from such pulp has long been desired.

An object of the invention is to provide the method of producing high yield pulp which can be used in the manufacture of paper having superior appearance and strength and other desirable properties.

We have found that the fiber bundles could be separated into individual fibers, by impregnating the pulp with an aqueous alkaline solution having a pH of 12 to 14, as measured at 25 C., and by subjecting the impregnated material to shearing stress in the common direction of the fiber axes in the bundle while it was simultaneously compressed substantially perpendicularly to the direction of the fiber axes.

Lignin and hemicellulose, which bond the individual fibers to each other in fiber bundles, swell by contact with an aqueous alkaline medium and are rendered to a plastic state. When a rearrangement between fiber bundles and fibers being in immediately adjacent position thereto in the alkaline medium is restricted by applying compressive stress substantially perpendicular to the direction of the fiber axes, shearing stress applied to the pulp substantially in said direction acts effectively and produces a strain locally on the swelled bonding area in the fiber bundles. The fibers are consequently disintegrated into individual fibers by mechanical failure in the bonding area. Simultaneously, the shearing force effects internal and external fibrillations of individual fibers, and therefore, the paper made from the treated pulp is superior in strength.

Compressive stress applied can be determined by a calculation based on the experimental results. When hot defibrated coarse pulp having a yield of about is used as material, the compressive stress required for the treatment according to the present invention is from 0.3 to 12 kg./cm. This value would vary, of course, depending upon the properties of the high yield coarse pulp to be treated and the details of production, for example, yield, cooking chemicals and defibration temperature employed for producing the coarse pulp. A quantity of shearing stress applied can vary depending upon such a condition of mechanical treatment as an applied power, and it should be sufficient at least to loosen the bond between the fibers in the fiber bundles.

Fiber bundles also cannot be seen in the treated pulp when compressive and shearing stresses have been applied to the pulp containing fiber bundles not in aqueous alkaline medium but in water. In the case of the mechanical treatment in water, however, the power requirement increases, fibers are broken and cut down to smaller pieces, and the paper made from the pulp treated in Water is far inferior in strength, as will be shown in Example 6.

An aqueous alkaline solution is used for the purpose of neutralizing acidic groups in the lignin and of swelling the lignin. The hydroxyl ion concentration, i.e. pH, of the solution is important for this invention, and the chemicals employed in the impregnating medium may be freely chosen among inorganic and organic compounds. The chemicals must be soluble in water and must yield the required hydroxyl ions to raise the pH value of the impregnating medium to the range of 12 to 14. This range is reached most economically under almost all conditions by means of sodium hydroxide, but the hydroxides of other alkali and alkaline earth metals are equally effective. Salts of strong bases with weak acids are also operative. Thus, sodium sulfide, trisodium phosphate, the corresponding potassium salts, and other salts of similar alkalinity may be used.

When high yield pulp is produced from wood by the conventional method, most of the lignin constituting the middle lamella is not removed, and all or some of said lignin, depending upon the producing conditions, remains on the surface of the individual fibers even after defibration. It has been found that the raw materials employed in this invention were high yield pulp whose defibrated fibers were covered over more than about 40% of their surface area with middle lamella and primary wall, and such pulp can be obtained by hot defibration. The lignin covering the fiber surface protects the cell walls of the fibers from suffering any damage by the alkaline impregnating solution and the mechanical stress employed during the treatment of this invention. Accordingly, fiber bundles in hot defibrated coarse pulp are separated into individual fibers by the method of this invention without suffering any longitudinal splitting or transverse breaking of the fibers, thus the decrease of freeness is relatively small.

The raw materials for the method of this invention are thus:

(1) Coarse pulp obtained by the steps of impregnating wood chips with water or with an aqueous cooking liquor having a pH of 1.0 to 12.7 measured at 25 C., and mechanically defibrating the impregnated chips at a temperature from 120 C. to 200 C., preferably from about 140 C. to about 180 C.;

(2) Coarse pulp obtained by the steps of impregnating wood chips with a cooking liquor containing an alkali metal sulfite and/ or an alkali metal bisulfite and mechanically defibrating the impregnated chips at a temperature from 120 C. to 200 C., preferably from about 140 C. to about 180 C., the content of the chemicals in said impregnated chips being from 1.0% to 30.0%, by weight, in terms of S based on the oven-dried chips.

When hot defibrated coarse pulp is pioduced according to the present invention, the kind of impregnating chemicals used is not so important but pH value of im pregnating liquor should be in the range of 1.0 to 12.7, preferably from about 4.0 to 12.7, measured at C. Extremely acid and alkaline conditions, i.e. outside said pH range, should be avoided, in view of the strength and brightness of the pulp after treatments by the alkaline solution and the mechanical stress. When the chips impregnated with an aqueous solution of sodium sulfite and/ or sodium bisulfite are defibrated, the coarse pulp so obtained is easily converted into the pulp containing no fiber bundles and having a high brightness according to the present invention. However, water without containing such chemicals or a conventional sulfite cooking liquor, for example, containing 5.0% free S0 and 1.2% combined S0 and having a pH of 1.5 can also be used as impregnating liquor.

The impregnated chips are heated at a high temperature for a short time and subjected to a hot defibration treatment. For example, the heating is conveniently carried out at a temperature of 140 C.180 C. for a time of 3 to 8 minutes and the defibration is carried out at the same temperature as the heating. The coarse pulp so obtained is suitable for use in the method of the present invention.

Hot defibrated coarse pulp is immersed in an aqueous alkaline solution having a pH range of 12.0 to 14.0, measured at 25 C. and, if desired after being squeezed, is subjected to the necessary mechanical treatment while the coarse pulp is present in the alkaline medium.

Hot defibrated coarse pulp of types (1) and (2) above has high resistance to the alkaline impregnating solution and the mechanical stress. When such kind of pulp are used, the mechanical treatment may be performed not only at high pulp consistency but also at low consistency. The treatment in high pulp consistency is preferred in view of avoiding any damage of the fibers, such as cutting, from the mechanical action.

The apparatus suitable for performing the mechanical treatment step of this invention includes the P.F.I. mill sold by the Imsets Co. of Oslo, Norway; the Ishikawa-type mixing and grinding machine sold by Kabushiki Kaisha Ishikawa Kojo, Tokyo, Japan; edge runners; and other apparatus having two coaxial discs driven independently at a peripheral velocity difference of less than 400 m./ sec.

Chemifiner sold by the Black-Clawson Co., Hamilton, Ohio, for example, may also be employed. Any other apparatus, which enables such compressive and shearing stresses to be effectively applied to the fiber bundles in coarse pulp may be employed successfully in this invention.

Even if the fibers in fiber bundles are not fully separated by the apparatus mentioned above, complete separation can be easily accomplished by a subsequent conventional beater or refiner treatment.

The following examples are further illustrative of this invention. Unless otherwise specifically stated, numerical values indicated in the examples were obtained according to Japanese Industrial Standards (J.I.S.). The values of area percentage of fiber bundles are determined in the following manner through the use of a microscope by measuring the area ratio of surface occupied by fiber bundles in the paper to that of the paper. The paper is prepared by a T.A.P.P.I. Standard hand sheet machine and has a basis weight of 15 g./m

EXAMPLE 1 606 g. basswod chips having an oven-dried Weight of 300 g. were immersed in 1974 ml. of an aqueous solution containing 25 g. of sodium sulfite and 15 g. of sodium bicarbonate per liter, evacuated at 10 mm. Hg to remove air from the chips and the evacuated condition was maintained at room temperature for 20 hours. The impregnated chips weighed 1010 g. which contained 9.4% the chemicals, by weight, on the basis of oven-dried chips. The impregnated chips were placed into a laboratory defibrator (Defibrator Corp., Stockholm, Sweden) and were heated to 153 C. (5.1 kg./cm. The temperature and pressure were maintained for eight minutes, whereupon the chips were defibrated under the same condition for two minutes.

After removal of knots, the coarse pulp (freeness, 770 cc., C.S.F.) was immersed into an aqueous sodium hydroxide solution having a pH of 13.2, the amount of said solution being 33 times, by Weight, of the coarse pulp. The coarse pulp was maintained in the aqueous alkaline solution at 25 C. for two hours, and divided into two then each pulp slurry was treated in a Lampen mill at a compressive force of 10 kg. for one and two hours, respectively (Batch I and II). The yield at this stage was 7580% on the basis of the oven-dried weight of wood chips employed as material. The treated pulp was washed with water and then prepared into paper having a basis weight of g./m. by T.A.P.P.I. Standard. For comparison, cold soda pulp was produced by the conventional process from the same kind of chips, and paper was prepared from the pulp as above (Batch III).

The properties of the paper so obtained are given in the following Table 1. From this table, it can be seen that paper prepared from cold soda pulp is much inferior in breaking length, burst factor and folding endurance in comparison with the paper produced from the pulp according to the method of this invention.

When the coarse pulp was treated in the Lampen mill for one hour according to the method of this invention, paper, having a basis weight of 50 g./rn. made therefrom, has a brightness of 53.6% and an opacity of 83.6%.

EXAMPLE 2 374 g. mixed wood chips made of six kinds of hard wood from Hokkaido, Japan, and having an oven-dried weight of 300 g. were immersed into 1726 ml. of an aqueous solution, containing 168 g. of sodium sulfite per liter and having a pH of 9.6, and after being evacuated, the immersion was continued for 20 hours as in Example 1. The impregnated chips weighed 782 g. and contained 23% of sodium sulfite, by weight, on the basis of oven-dried chips. The impregnated chips were heated at 148 C. and 4.5 kg./cm. for eight minutes and defibrated at that temperature for two minutes. The yield after defibration was 88.1%.

79 g. (30 g. in oven-dried weight) of the coarse pulp so obtained were immersed in 921 ml. of an aqueous solution, containing 7.38 g. of sodium hydroxide per liter of the solution and having a pH of 13.1, for one hour. The pulp slurry was then treated in an Ishikawa-type mixing and grinding machine (Model No. 18) under a compressive stress of 12 kg./cm. for four hours. After washed with water, the pulp was prepared into paper having a basis weight of 67 g./m. The properties of the paper so obtained are given in the following Table 2.

TABLE 2 Properties of paper stock:

'Freeness (C.S.F., cc.) 295 Area percentage of fiber bundles (percent) Properties of paper:

Density (g./cm. 0.79 Breaking length (km.) 6.6 Burst factor 3.9

Tear factor 48 Folding endurance (number of folds) 48 EXAMPLE 3 362 g. wood chips as in Example 2 having an ovendried weight of 300 g. were immersed into 1738 ml. of an aqueous solution containing 82 g. of sodium sulfite per liter, and after being evacuated, the immersion was continued for 20 hours as in Example 1. The chips weighed 780 g. and contained 13% of sodium sulfite, by weight, on the basis of oven-dried chips. The impregnated chips were heated at 148 C. and 4.5 kg./cm. for eight minutes and defibrated at the same temperature for two minutes. The yield after de fibration was 87.3%.

This coarse pulp having a freeness of 780 cc. (C.S.F.) was immersed in an aqueous sodium hydroxide solution having a pH of 13.1 at room tempertaure for one hour, the weight of said solution being times that of the pulp. The pulp was squeezed to a consistency of 29.6% and then subjected repeatedly three times to the treatment by a Chemifiner, type D16. This apparatus comprised two opposing discs, which were in an arrangement of an eccentric relation of 10 mm. and of a clearance of 0.5 mm. therebetween, and in which main disc was positively driven at 970 r.p.m., while another disc was a follower adapted to be driven from the main disc through the material passing between the discs. The pulp treated with the Chemifiner had a freeness of 605 cc. (C.S.F.) and was diluted to a consistency of 3%. It was then beaten in a test beater for 35 minutes and prepared into paper by T.A.P.P.l. Standard (Batch lV). The results are given in the following Table On the other hand, for the purpose of comparison, two kinds of kraft pulp, produced from soft wood (Batch V) and several kinds of hard wood from Hokkaido, Japan (Batch VI), were treated in accordance with procedures similar to the above experiment using the same Chemifiner except that the treatment was carried out in water. Variations in the condition of the Chemifiner are given in Table 4 which also lists the freeness of the pulps before and after Chemifiner treatment.

TABLE 3 Properties of paper stock (after beating): Freeness (C.S.F. cc.) 325 Area percentage of fiber bundles (percent) 0.2 Properties of paper:

Basis weight (g./m. 68.5 Breaking length (km.) 3.6 Burst factor 2.2 Tear factor 47 Folding endurance (number of folds) 6 As is evident from Table 4, it has been found that the freeness of the pulp treated according to the method of this invention was decreased through the treatment by the Chemifiner, while the freeness of kraft pulp was increased through the similar treatment by the same Chemi-finer.

EXAMPLE 4 Mixed chips (water content, 465%) of hard wood from Hokkaido, Japan, were carried on a belt conveyer at a rate of 1.2 metric tons (oven-dried weight basis) per hour. An aqueous solution of to 50 C., containing 129 g. of sodium sulfite per liter of the solution, was sprayed on the chips and 7.5%, by weight for oven-dried chips, of sodium sulfite was impregnated. The chips so treated were heated in a commercial horizontal Asplund digester for 7 minutes at 170 C. and 7.0 kg./cm. and were defibrated at the same temperature and pressure. A pH value before second defibration was 4.8. They were then subjected to a second, light defibration treatment with a conventional disc refiner at a temperature of about C. The coarse pulp so obtained in a yield of 89% was inspected under an electron microscope, and it was found that 99% or more of the surface area of the fibers was covered with middle lamella and/or primary wall.

The coarse pulp having a freeness of 770 cc. (C.S.F.) was immersed in a 0.6% sodium hydroxide solution (pH 13.0) for one hour in 4% pulp consistency, and was squeezed to a 25.4% pulp consistency. Thereafter the impregnated pulp was treated once with a Bauer double disc refiner (36" disc diameter, 1460 rpm. and 400 HP) under a compressive stress of 0.9 kg./cm. at m./sec. or" peripheral velocity difference between the discs.

The treated pulp, which had a freeness of 645 cc. (C.S.F), was beaten in. at Valley heater and prepared into hand sheets according to T.A.'P.P.l Standard. The properties of the paper are given in Table 5.

TABLE Properties of paper stock (after beating) Freeness (C.S.F., cc.) 285 Area percentage of fiber bundles (percent) 0.2

Properties of paper:

Basis weight (g./m. 66.3

Breaking length (km.) 4.9 Burst factor 3.0

Tear factor 50 Folding endurance (number of folds) EXAMPLE 5 Philippine mahogany chips (water content: 43%) were carried on a belt conveyer at a rate of 1.84 metric tons (oven-dried weight basis) per hour. An aqueous solution of to C., containing 215 g. of sodium sulfite per liter of the solution, was sprayed on the chips and 6.7%, by weight for oven-dried chips, of sodium sulfite was impregnated. The chips so treated were heated in a commercial horizontal Asplund digester for 7 minutes at 168 C. and 6.5 to 7.0 kg./cm. and were defibrated at the same temperature and pressure. They were then subjected to a second, light defibration treatment with a conventional disc refiner at a temperature of about C. The coarse pulp so obtained in a yield of was inspected under an electron microscope, and it was found that or more of the surface area of the fibers was covered with middle lamella and/or primary wall.

The coarse pulp having a freeness of 780 cc. (C.S.F) was immersed in a 0.65% sodium hydroxide solution (pH 13.1) for one hour in a 4% pulp consistency, and was squeezed to a 25% pulp consistency. Thereafter the impregnated pulp was treated twice with a Bauer double disc refiner (36" disc diameter, 1460 rpm. and 400 HP) at m./sec. of peripheral velocity difierence between the discs.

The so treated pulp, which had a freeness of 576 cc. (C.S.F), was bleached with sodium hypochlorite, beaten in a Valley beater and was prepared into hand sheets according to T.A.P.P.I. Standard. The properties of the paper are given in Table 6.

TABLE 6 Properties of paper stock (after beating) Freeness (C.S.F., cc.) 280 Area percentage of fiber bundles (percent) 0.3 Properties of paper:

Basis weight (g./m. 63.4

Breaking length (km.) 3.2

Burst factor 1.5

Tear factor 42 Folding endurance (number of folds) 3 Brightness (Hunter, percent) 55 EXAMPLE 6 Coarse pulp was prepared as in Example 2, except that the chips contained 21% of the chemicals and the yield was 83.9%. The coarse pulp was immersed in an aqueous sodium hydroxide solution having a pH of 13.0 for one hour at room temperature, the amount of said solution being 6.7 times, by weight, of the pulp. The pulp slurry was squeezed to remove excess sodium hydroxide solution so that the resulting material had a consistency of 30%. It was then treated for 42 minutes in a P.F.I. mill on which the difference of peripheral velocities of the roll and of the cylindrical casing was nearly zero, the clearance between the roll and the casing was 0.2 mm. and the treating pressure was 3.4 kg./cm. corresponding to a compressive stress of 6.8 kg./cm. (a 'bar width of the roll is 0.5 cm.). Paper was made from the treated pulp (Batch VII).

The properties of the pulp and of the paper are listed in Table 7 together with corresponding values obtained from a batch of the same coarse pulp which was immersed in water (pH 6.0) instead of the sodium hydroxide solution (Batch VIII).

the treatment. Properties of paper:

Basis Weight (g./n1. Breaking length (km.) Burst fact Tear factor.

Folding endurance (number of 22 2 folds).

Brightness (Hunter, percent) 49 54.

The fiber bundles could hardly 'be seen in the pulp after the P.F.I. mill treatment. When water was used for the immersion or impregnating treatment preceding the mechanical treatment, the freeness of the pulp was reduced, but the paper prepared from the pulp treated in water was far inferior in strength to those prepared from the material treated in the alkaline medium.

Observation under an optical microscope showed a large diiference in the morphology of fibers between treatments in water and in the alkaline medium, and it was found that fibers treated in water were broken and cut down to smaller pieces.

EXAMPLE 7 Mixed chips (Water content, 45.5%) of hard Wood from Hokkaido, Japan, were carried on a belt eonveyer at a rate of 1.34 metric tons (oven-dried weight basis) per hour. An aqueous solution of 55 C. to 60 C., containing 132 g. of sodium sulfite per liter of the solution, was sprayed on the chips and 6.9%, by weight, for ovendried chips, of sodium sulfite was impregnated. The chips so treated were heated in a commercial horizontal Asplund digester at 7.0 kg./cm. and C. for 7 minutes, and were defibrated at that pressure and temperature. They were then subjected to a second, light defibration treatment with a disc refiner at a temperature of about 60 C. The coarse pulp so obtained was inspected under an electron microscope, and it was found that about 90% of the surface area of the fibers was covered with middle lamella and primary wall.

The coarse pulp having a freeness of 768 cc. (C.S.F.) was immersed in aqueous alkaline solutions of sodium hydroxide and of potassium hydroxide, having a pH of 13.3, for 30 minutes at a 10% pulp consistency, respectively. After being squeezed to a 25 pulp consistency, the coarse pulp (oven-dried weight was 22.5 g.) was treated in a P.F.I. mill, and prepared into hand sheets by T.A.P.P.I. Standard. The set conditions in the P.F.I. mill were 0.2 mm. clearance between the treating surfaces, 3.4 kg./cm. treating pressure corresponding to a compressive stress of 6.8 kg/cm. and 1.9 m./sec. peripheral velocity difference between the roll and the cylindrical casing. The properties of the papers prepared are listed in Table 8.

TABLE 8 Chemical NaOH KOH P.F.I. mill treatment:

pH of treating medium at the start of treatment at 25 C 12. 8 12.9 Treating time (min) 10. 5 9. 8 Freeness (C.S.F. ee.);

Before treatment 768 768 After treatment 265 268 Area percentage of fiber bundles after the treatment (percent) 0.7 O. 5 Properties of paper:

Basis weight (g./m. 65.3 64. 7 Breaking length (km) 3. 7 3. 6 Burst factor 2.1 2.0 Tear factor 44 45 Folding endurance (number of folds) 6 5 9 EXAMPLE 8 373 g. wood chips as in Example 2 having an ovendried weight of 300 g. were immersed into water at a temperature of 96 C. for three minutes, and excess water was drained off. The pretreated chips were immersed into 2927 ml. of an aqueous solution containing 25 g. of sodium bisulfite and 134 g. of sodium sulfite per liter and having a pH of 6.9. The chips in the solution were compressed under a pressure of 140 kg./cm. at a temperature of 70 C. and then immediately followed by an impregnation under atmospheric pressure for 10 minutes. The impregnated chips weighed 655 g. which contained 15.0% the chemicals, by weight, on the basis of oven-dried chips. The impregnated chips were heated at 148 C. and 4.5 kg./cm. for eight minutes and defibrated at that temperature for two minutes. The yield after defibration was 85.8%.

After removal of knots, the coarse pulp (freeness, 780 cc. (C.S.F. was immersed into an aqueous sodium hydroxide solution having a pH of 13.1, the amount of said solution being 33 times, by weight, of the coarse pulp. The coarse pulp was maintained in the aqueous alkaline solution at 25 C. for two hours, and then the pulp slurry was treated in a Lampen mill at a compressive force of 10 kg. for three hours and thirty minutes. The treated pulp was washed with water and then prepared into paper by T.A.P.P.I. Standard.

The properties of the paper so obtained are given in the following Table 9.

TABLE 9 Properties of paper stock:

Freeness (C.S.F., cc.) 310 Area percentage of fiber bundles (percent) Properties of paper:

Basis weight (gm?) 65.7 Breaking length (km.) 3.9 Burst factor 1.7 Tear factor 36 Fold endurance (number of folds) 2 EXAMPLE 9 Mixed chips (water content, 43%) of hard wood from Hokkaido, Japan, were carried on a belt conveyer at a rate of 1.3 metric tons (oven-dried weight basis) per hour. The chips were heated in a commercial horizontal Asplund digester for 3 minutes at 175 C. and 8.0 kg./ cm. and were defibrated at the same temperature and pressure. They were then subjected to a second, light defibration treatment with a conventional disc refiner at a temperature of about 60 C.

This coarse pulp was immersed in an aqueous sodium hydroxide solution having a pH of 13.4 at room temperature for one hour, the weight of said solution being 10 times that of the pulp. The pulp was squeezed to a consistency of 29.3% and then subjected repeatedly three times to the treatment by a Chemifiner, type D16. This apparatus comprised two opposing discs, which were in an arrangement of an eccentric relation of mm. and of a clearance of 0.5 mm. therebetween, and in which main disc was positively driven at 970 r.p.rn. The pulp treated with the Chemifiner had a freeness of 533 cc. (C.S.F.) and was diluted to a consistency of 3%. It was then beaten in a test heater for 22 minutes and prepared into paper by T.A.P.P.I. Standard. The results are given in the following Table 10.

10 TABLE 10 Properties of paper stock (after beating) Freeness (C.S.F., cc.) 320 Area percentage of fiber bundles (percent) 0.6 Properties of paper:

Basis weight (g./m. 67.6

Breaking length (km.) 3.0

Burst factor 1.7

Tear factor 38 Folding endurance (number of folds) 5 Brightness (Hunter, percent) 33 EXAMPLE 10 375 g. mixed wood chips made of six kinds of hard wood from Hokkaido, Japan, and having an oven-dried weight of 300 g. were immersed into 2325 ml. of an aqueous solution, containing 51.6 g. of sodium sulfite and 11.3 g. of formaldehyde per liter and having a pH of 12.7 at 25 C., and after being evacuated, the immersion was continued for 20 hours as in Example 1. The impregnated chips weighed 798 g. The impregnated chips were heated at 148 C. and 4.5 kg./cm. for eight minutes and defibrated at that temperature for two minutes. The yield after defibration was 83.3%.

79 g. (30 g. in oven-dried weight) of the coarse pulp so obtained were immersed in 921 m1. of an aqueous solution, containing 1.05 g. of sodium hydroxide per liter of the solution and having a pH of 12.3, for one hour. The pulp slurry was then treated in an Ishikawa-type mixing and grinding machine (Model No. 18) for three hours and ten minutes. After washing with water, the pulp was prepared into paper having a basis weight of 65.7 g./m. The properties of the paper so obtained are given in the following Table 11.

TABLE 11.

Properties of paper stock:

Freeness (C.S.F., cc.) 300 Area percentage of fiber bundles (percent) 0.3 Properties of paper:

Density (g./cm. 0.73

Breaking length (km.) 5.7 Burst factor 3.4 Tear factor 54 Folding endurance (number of folds) 33 While several preferred embodiments of the invention have been described in detail, it should be understood that they are given for the purpose of illustration only and are not to be regarded as a definition on the scope of the invention, reference being had for this purpose to the appended claims.

What is claimed is:

1. A method of producing high yield pulp, which comprises:

(a) impregnating wood chips with a treating liquid having a pH of 1.0 to 12.7, when measured at 25 C., and mechanically defibrating the impregnated chips at a temperature between C. and 200 C.;

(b) immersing the defibrated material in an alkaline aqueous medium having a pH of from 12 to 14, as measured at 25 C., and impregnating the material with said medium;

(0) treating the impregnated material with a mechanical refining apparatus to apply compressive stress substantially perpendicularly to the common direction of the fiber axes in the fiber bundles, said compressive stress being from 0.3 kg./cm. to 12 kg./ cm. and simultaneously to apply shearing stress substantially in said common direction, thereby separating the fiber bundles into individual fibers.

2. A method as set forth in claim 1, wherein said liquor is an aqueous solution of an alkali metal sulfite, or of an alkali metal bisulfite, or of a mixture of an alkali metal sulfite with an alkali metal bisulfite, the concentration of said sulfite, said bisulfite, or said mixture being 1 1 l2 s suflicient to make the etfective concentration of sulfur di- References Cited oxide in the impregnated chips between 1.0% and 30.0% of the oven-dried weight of said chips. UNITED STATES PATENTS 3. A method as set forth in claim 1, wherein said com- 3 186 99 19 5 Madison 1 2 2 pressive stress and said shearing stress are applied to said 5 3 3 3 037 195 Asplund bundles by treating said impregnated material with a disc refiner having two discs, said discs differing in peripheral HOWARD R, CAINE, P i E i velocity by more than 0 meter per second but not more than 400 meters per second while acting on said mate- US. Cl. X.R.

rial. 10 16226, 86 

